This project involves theoretical and empirical studies which aim to contribute to developing a socio-economic theory of fertility that will encompass better than current theories the concepts and empirical results of fertility research in several disciplines, especially economics and sociology, and be applicable to the study of fertility in both pre-modern and modern societies. The theoretical studies focus, among other things, on the treatment within an economics framework of preferences, costs of fertility regulation, and natural fertility. Three empirical studies are planned. Two extend previous work of the principal investigators -- one on rural fertility in new agricultural areas in mid-nineteenth century United States and one on American fertility and labor market experience in recent decades -- and one, based on data source comprising a panel of 16,000 pairs of white male twins, is new. All three share in common further testing of the "relative income" hypothesis, but each has distinctive features as well. For example, the rural fertility study will extend the earlier approach to a less developed area; the analysis of recent experience will seek to make relative income endogenous to the model; and the "twin" study offers the possibility of new work on the determinants of preferences.